Last week, Peter Oborne contacted 10 Downing Street to ask the press office to substantiate David Cameron’s statement, made at Prime Minster's Questions on 20 April, that the London imam Suliman Gani was a supporter of the Islamic State group. Despite several requests, the press office refused to comment.

Peter Oborne was referred to a political spokesman but he too was unable to provide any evidence. We publish below an open letter from Peter Oborne to Katherine Allen, a spokeswoman for the Prime Minister’s Press Office.

Dear Ms Allen,

Thank you for your email of 27 April, in which you told me that the Downing Street press office was not prepared to answer questions about David Cameron’s claim at Prime Minister’s Questions that the Tooting Imam Suliman Gani "supports IS".

You said that this was not a matter for the Downing Street press office, and directed me to the prime minister’s political team. I am afraid this refusal to help suggests that you do not fully understand your duties as a Downing Street press spokeswoman. In this email I am appealing to you to change your mind:

To deal first with the narrow point: your refusal to answer my questions on the grounds that civil servants do not comment on political statements. I accept that the No 10 Press Office is not responsible for comments made by the prime minister on the campaign trail. However, I would remind you that David Cameron made his statement concerning Mr Gani in his capacity as prime minister, while discharging his official responsibilities in the House of Commons. It is up to the Downing Street press office to account for statements made by the prime minister in this official context.

You and your colleague excused yourselves from answering questions on the grounds that the prime minister was making a "political" statement about Mr Gani. The implications of this are shocking and outrageous. Are you trying to tell me that the prime minister’s statement to parliament that Mr Gani supported IS was a political (ie partisan) point? This would be grotesque.

Expressing support for the Islamic State is a criminal offence (only last week Mohammad Mohsin Ameen was sentenced to five years in jail for inviting support for IS). Apart from anything else, the prime minister might like to consider that his attack risks putting Mr Gani in harm’s way by opening him up to reprisals. It is wholly unacceptable that a British prime minister should smear a British citizen in this way as part of a political campaign to damage the Labour candidate for London mayor.

Although you refused to comment on the prime minister’s statement, your office did send me a link to a news story (Sky News, 20 April 2016 "PM Branded ‘Racist’ for Attack on Labour’s Khan") which contained a quote from a "Downing Street spokesman". The quote reads as follows: "The prime minister makes no apology for raising very serious concerns about people Sadiq Khan has shared a platform with. Suliman Gani, on the night of the Paris terror attacks, was in Bedford calling for an Islamic state."

This is very troubling indeed. On the night of the Paris attacks Mr Gani was in Bedford where he attended a public meeting called "Quiz-a-Muslim". There is a recording of this meeting on YouTube. I have studied it. At the meeting Mr Gani said nothing which could be construed as expressing support for IS.

Why did the Downing Street spokesman draw attention to the fact that Mr Gani’s Bedford meeting took place on the night of Paris attacks? This is a disgraceful reference because it can only have been made in order to suggest a connection between the Bedford event and the tragedy in Paris - when no such connection has ever been made and none to my knowledge exists.

In the light of the prime minister’s intervention at PMQs I interviewed Mr Gani. He told me that he condemns IS and has campaigned against IS. He also showed me citations, press articles and other materials showing that he has worked alongside a local synagogue and with local churches. This is not the behavior of someone who supports IS.

To sum up, the prime minister has made a very serious charge against a British citizen – namely that that he supports IS, the most notorious terror group in the world. Neither he nor his spokespeople (ie you and your colleagues, Ms Allen) have been able to provide a scintilla of evidence to support this claim. Since he is unable to provide the evidence, I believe the prime minister must urgently withdraw his false and profoundly damaging assertion about Mr Gani.

Could I remind you of paragraph 1.2.c of the Ministerial Code. It states that: “It is of paramount importance that ministers give accurate and truthful information to Parliament, correcting any inadvertent error at the earliest opportunity. Ministers who knowingly mislead Parliament will be expected to offer their resignation.” The prime minister will be familiar with the contents of the Ministerial Code, since he wrote a foreword to it.

It is essential that the prime minister acts as quickly as possible. As you yourself seem to imply by directing me to the prime minister’s political office, he made this allegation against Mr Gani as a partisan ploy designed to gain votes for the Tory candidate Zac Goldsmith in the London elections. It would be utterly discreditable – indeed damnable - if the prime minister’s slur against Mr Gani were to remain on the record. He must either substantiate it, which I am certain he cannot do - or withdraw it, with a full apology

I am publishing this as an open letter in the Middle East Eye, where I write a column.

Yours sincerely

Peter Oborne

- Peter Oborne was British Press Awards Columnist of the Year 2013. He recently resigned as chief political columnist of the Daily Telegraph. His books include The Triumph of the Political Class; The Rise of Political Lying;and Why the West is Wrong about Nuclear Iran.

The views expressed in this article belong to the author and do not necessarily reflect the editorial policy of Middle East Eye.